This invention relates generally to improvements in electromechanical acoustic emission detectors or transducers for detecting acoustic emission signals that are emitted by solid objects with those signals being caused, for example, by cracking or failure in some way in the solid object of interest, and converting the signals obtained into electrical signals having components of corresponding frequency. The electrical signals are converted, for example, into computer readouts for determining the degree of acoustic emission signal being emitted. This invention relates particularly to an improvement in an arrangement of acoustic emission transducer for measuring surface displacement waveforms as developed by the National Bureau of Standards (NBS) and reported in their report of December, 1981, entitled "Information Packet on the NBS Conical Transducer for Acoustic Emission," which report is incorporated by reference in its entirety herein.
The phenomenon of acoustic emission, to which the present invention relates, is concerned with the detection of elastic waves that are emitted from a source within an object and become manifest at positions remote from the source. Such waves are developed, for example, in the cracking of a pressure vessel shell either internally, or on the surface when it is failing or deteriorating, or in failure in the welding of joints of the plates or other parts making up or forming the device of interest. Acoustic emission signals are also emitted from various points of an object of interest spontaneously at various times. However, generally acoustic emission occurs as a result of growth of cracks in a device, in failing, and such cracks may arise from application of various kinds of forces including mechanical pressure or temperature changes, or even by deterioration caused by chemical action on the structure involved.
The location of cracks and other sources of acoustic emission has been determined by acoustic emission detector arrays for many years generally by locating acoustic emission detectors at various points on the surface of the structure being investigated, detecting the signals received and the relative times of arrival of the signals to the various respective detectors, and then either manually or automatically calculating the origin of the acoustic emission signals by triangulation or similar methods, taking into account the relative times of arrival of the stress waves, for example, being detected at the detectors and the propagation speed of those waves. The severity of the source of the acoustic emission signal is determined by observing the strength of the waves arriving at the detectors such as, for example, by counting the number of pulses or groups of pulses detected in a given interval of time or over an extended time.
As will be appreciated, in the environment of the object under consideration, many other sound generated signals may occur simultaneously in the operation of the object, for example, or in the operation of equipment adjacent to the object under consideration. These signals constitute noise, and therefore, if also detected, may obscure the acoustic emission signals being read. For this reason, various methods have been developed for improving the signal to noise ratio, and many arrangements of apparatus have been devised to enhance the pick-up of the signals of interest developed from acoustic emissions. It is to this application that the present invention is directed of an improved transducer for sensing surface flat frequency response displacement waveforms which is packaged to provide a device of rugged construction, which may be handled easily in any orientation. The device is much smaller in size and easier to use because the signal is substantially less distorted and more information is obtainable.
The present invention includes a housing for mounting the transducer instrument with that instrument including a sensor element in the form of a piezoelectric crystal, with that crystal being mounted in a housing under a continuously applied bias pressure and the use of an inertia mass of high mechanical impedence, and of sufficient size to clamp one face of the crystal from the high frequency motions experienced by the other face of the crystal. With such an arrangement, it has been established that the voltage output of the crystal will be proportional to the displacement of the other face of the crystal. Included in the housing, on the opposite side of the mass from the crystal, is an elastomer bulk for holding the inertia mass in place, absorbing jolts, and for continuously applying the desired force or pressure against the crystal.
In those instances where the structure under consideration is steel, which is a very common structural material for pressure vessels and structures where acoustic emission signals are of interest, the housing for the instrument, in accordance with this invention, is comprised of stainless steel, and preferably corrosion-resistant stainless steel. Also it may be non-magnetic and austenitic. Such material is easy to weld and to machine into the very accurate dimensions required for the instrument of the invention. The inertia mass is comprised of a material selected to have the highest available mechanical impedence of the crystal utilized as the piezoelectric sensor element. Preferably, the inertia mass is comprised of a tungsten alloy such as a tungsten-carbide alloy.
Material suitable for use as piezoelectric elements include ceramic polycrystalline materials which are polarizable, as well as natural piezoelectric crystals. Such ceramic materials include barium titanate, lead-zirconate, lead metaniobate, bismuth titanate, and mixtures thereof with each other or with other suitable materials. Natural crystals for use in the invention include quartz and lithium niobate, etc. Preferably, the crystal utilized in accordance with this invention is lead-zirconate-titanate.
The crystal may have several forms of shape, including a plate-like form, a bar form, or a truncated cone as with the NBS device. Preferably with this invention, the crystal is a plate form, cylindrical in shape, with a diameter of substantially about 0.05 inches and a thickness of substantially about 0.025 inches.
In connection with the development of an instrument for use according to the invention here, in combination with the lead-zirconate-titanate combination sensor crystal, it has been found that the inertia mass is preferably comprised of Densalloy, a tungsten base heavy alloy, and a product of Teledyne Powder Alloys of Clifton, N.J.
This invention will now be described in more detail and other objects and advantages thereof will become apparent from the following description, the accompanying drawings, and the appended claims.